Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been adopted for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the cell. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by lower than expected energy storage performance (e.g., round trip energy efficiency) and poor cycle life, among other factors. Despite significant development efforts, no commercially viable flow battery technologies have yet been developed.
In many instances, iron hexacyanide complexes can be highly desirable for use as active materials in flow batteries and other electrochemical energy storage systems. These complexes exhibit facile electrode kinetics and reversible electrochemical behavior at redox potentials near the oxidative thermodynamic stability limit of aqueous solutions. Further, these complexes are composed of abundant elements and are not overly expensive. As used herein, the term “iron hexacyanide complex” will refer to the oxidation-reduction couple of ferrocyanide (i.e., Fe(CN)64−) and ferricyanide (i.e., Fe(CN)63−). These complex ions can be present in any combination where the content of ferrocyanide and ferricyanide sums to 100%, including instances where there is 100% ferrocyanide or 100% ferricyanide. The compositional extremes represent a state of full discharge or full charge, depending upon the half-cell in which the active material is present. Various counterions can complete the charge balance of the iron hexacyanide complexes.
Despite the well understood and desirable oxidation-reduction behavior of iron hexacyanide complexes, these complexes unfortunately exhibit relatively limited solubility in aqueous solutions, thereby leading to low energy densities. Further, unwanted precipitation of the active material can occur if the electrolyte solution is near its saturation concentration. For flow batteries, in particular, it can be desirable to utilize an active material concentration that is somewhat removed from the saturation concentration to decrease the risk of unwanted precipitation and potential occlusion of circulation pathways and other components within the flow battery. This can further decrease the energy density.
Although the low solubility of iron hexacyanide complexes can be mitigated to some degree based upon the identity of the counterion, solubility often still remains a prevalent concern. Moreover, many of the counterions that improve solubility for iron hexacyanide complexes can exhibit varying degrees of incompatibility with certain flow battery components. For example, calcium and other divalent counterions can dramatically improve the solubility of iron hexacyanide complexes, but such counterions can be fouling toward thin membranes separating the flow battery's half-cells, thereby diminishing performance. Moreover, many divalent metal ions are prone toward formation of insoluble hydroxides under alkaline conditions.
For a variety of reasons, it can be desirable to solubilize iron hexacyanide complexes under alkaline conditions. Among other reasons, potential reactivity of the cyanide ligands with acid can be averted. Since the pH conditions in an electrolyte solution can frequently change over the course of repeated charging and discharging cycles (e.g., due to parasitic reactions), it can be desirable to include a buffer in the electrolyte solution to resist a potentially detrimental change in pH. While buffers can indeed help resist unwanted pH changes in an electrolyte solution, the dissolved buffer material can undesirably decrease the saturation concentration of the iron hexacyanide complex and further complicate an already challenging solubility profile. The decreased iron hexacyanide complex solubility of buffered electrolyte solutions can be particularly problematic in large-scale energy storage applications where high energy densities are desirable.
In view of the foregoing, buffered electrolyte solutions containing high concentrations of dissolved iron hexacyanide complexes would be highly desirable in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.